Iodine [I]

Characteristics

An: 53 N: 74
Am: 126.90447 (3) g/mol
Group No: 17
Group Name: Halogen
Block: p-block Period: 5
State: solid at 298 K
Colour: Violet-dark grey, lustrous
Classification: Non-metallic
Boiling Point: 457.4K (184.3oC)
Melting Point: 386.85K (113.7oC)
Critical temperature: 819K (546oC)
Density: 4.933g/cm3

Discovery Information

Who: Bernard Courtois
When: 1804
Where: France

Name Origin

Greek: iodes (violet). "Iodine" in different languages.

Sources

Occurs on land and in the sea in sodium and potassium compounds. Although the element is actually quite rare, kelp and certain other plants have the ability to concentrate Iodine, which helps introduce the element into the food chain as well as keeping its cost down.


When heated Iodine sublimates (turns from a solid to a gas) into a violet vapour.
Primary producers are Chile (c.66%) and Japan and the USA. Annual production is around 12 thousand tons.

Abundance

Universe: 0.0001 ppm (by weight)
Carbonaceous meteorite: 0.26 ppm
Earth’s Crust: 1.4 ppm
Seawater: Atlantic surface: 4.89 x 10-2 ppm; Atlantic deep: 5.6 x 10-2 ppm; Pacific surface: 4.3 x 10-2 ppm; Pacific deep: 5.8 x 10-2 ppm
Human: 200 ppb by weight; 10 ppb by atoms

Uses

Required in small amounts by humans. Once used as an antiseptic, but no longer due to its poisonous nature. Silver iodide (AgI) is used in photography. Tungsten iodide is used to stabilise the filaments in light bulbs. Iodine-131 is used as a tracer in medicine.

Potassium iodide (KI tablets, or "SSKI" = "Super-Saturated KI" liquid drops) can be given to people in a nuclear disaster area when fission has taken place, to flush out the radioactive iodine-131 fission product. The half-life of iodine-131 is only eight days, so the treatment would need to continue only a couple of weeks. In cases of leakage of certain nuclear materials without fission, or certain types of dirty bomb made with other than radioiodine, this precaution would be of no avail.

Tungsten iodide (WI) is used to stabilize the filaments in light bulbs.

Iodine-123 and iodine-125 are used in medicine as tracers for imaging and evaluating the function of the thyroid.

Iodine-131 is used in medicine for treatment of thyroid cancer and Grave’s disease.

History

Iodine was discovered by Bernard Courtois in 1811. He was born to a manufacturer of saltpeter (a vital part of gunpowder). At the time France was at war, saltpeter, a component of gunpowder, was in great demand. Saltpeter produced from French niter beds required sodium carbonate, which could be isolated from seaweed washed up on the coasts of Normandy and Brittany. To isolate the sodium carbonate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding sulfuric acid. One day Courtois added too much sulfuric acid and a cloud of purple vapour rose. Courtois noted that the vapour crystallized on cold surfaces making dark crystals. Courtois suspected that this was a new element but lacked the money to pursue his observations.

However he gave samples to his friends, Charles Bernard Desormes (1777 - 1862) and Nicolas Clement (1779 - 1841), to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac (1778 - 1850), a well-known chemist at that time, and to Andre-Marie Ampere (1775 - 1836). On 29 November 1813, Dersormes and Clement made public Courtois’ discovery. They described the substance to a meeting of the Imperial Institute of France. On December 6, Gay-Lussac announced that the new substance was either an element or a compound of oxygen
. Ampere had given some of his sample to Humphry Davy (1778 - 1829). Davy did some experiments on the substance and noted its similarity to chlorine. Davy sent a letter dated December 10 to the Royal Society of London stating that he had identified a new element. A large argument erupted between Davy and Gay-Lussac over who identified http://jejaringkimia.blogspot.com/2011/08/iodine-i.html first but both scientists acknowledged Bernard Courtois as the first to isolate the chemical element.

Notes

It is an essential trace element; the thyroid hormones, thyroxine and triiodothyronine contain Iodine.
Iodine is a dark-gray/purple-black solid that sublimes at standard temperatures into a purple-pink gas that has an irritating odour. This halogen forms compounds with many elements, but is less active than the other members of the halogens and has some metallic-like properties.

Hazards

Toxic, many be fatal is swallowed or inhaled. Direct contact with skin can cause lesions, so it should be handled with care. Iodine vapour is very irritating to the eye and to mucous membranes.
When mixed with ammonia (NH3) it can form nitrogen triiodide (NI3) which is extremely sensitive and can explode unexpectedly.

Iodine Compounds

Ammonium iodide NH4I
It is used in photographic chemicals and some medications.

Iodic acid HIO3
Iodic acid is used as a standard strong acid in analytical chemistry. It may be used to standardize solutions of both weak and strong bases, with methyl red or methyl orange as the indicator.

Lead(II) iodide PbI2 [ Toxic ]
As toxic, yellowish solid. In its crystalline form it is used as a detector material for high energy photons including x-rays and gamma rays.

Lithium iodide LiI
Lithium iodide is used as an electrolyte for high temperature batteries. It is also used for long life batteries as required, for example, by cardiac pacemakers. The solid is used as a phosphor for neutron detection.

Nitrogen triiodide NI3
Also called nitrogen iodide, is a highly explosive compound of nitrogen and Iodine
. It is a contact explosive, and small quantities explode with a gunpowder-like snap when touched by even a feather, releasing a volatile cloud of Iodine vapour.
Small amounts of nitrogen triiodide are sometimes synthesized as a demonstration to chemistry students. However, because the compound is so unstable, it has not been used in blasting caps or primers for explosives.
The reason for it’s instability is due to the size difference between the two different types of atoms. The three Iodine atoms are much bigger than the nitrogen atom holding them together. Because of this, not only is the bond between nuclei under much stress and therefore weaker, but the outside electrons of the different iodine atoms are very close, which increases the overall instability of the molecule.

Potassium iodide KI
Potassium iodide is used in photography, in the preparation of silver(I) iodide for high speed photographic film.

Potassium iodide may also be used to protect the thyroid from radioactive iodide in the event of an accident or terrorist attack at a nuclear power plant, or other nuclear attack, especially where a nuclear reactor is breached and the volatile radionuclides, which contain significant amount of 131I, are released into the environment. Radioiodine is a particularly dangerous radionuclide because the body concentrates it in the thyroid gland.

Sodium iodide NaI
Sodium iodide is commonly used to treat and prevent iodine deficiency. Solid crystals can be used to detect radiation (e.g. radiation from uranium) - a solid crystal of sodium iodide creates a pulse of light when radiation interacts with it.

Thyroxine (T4) 3,5,3’,5’-tetra-iodothyronine
An important hormone produced by the thyroid gland.
The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds. Numerous physiological and pathological stimuli influence thyroid hormone synthesis.
Hypothyroidism is a disorder where there is a deficiency of thyroxine. Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both.

Triiodothyronine (T3)
An important hormone produced by the thyroid gland.

The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds. Numerous physiological and pathological stimuli influence thyroid hormone synthesis.
Hypothyroidism is a disorder where there is a deficiency of thyroxine. Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both.

Reactions of Iodine

Reactions with water
Iodine reacts with water to produce hypoiolite, OI-. The pH of the solution determines the position of the equilibrium.
I2(l) + H2O(l) --> OI-(aq) + 2H+(aq) + I-(aq)
 
Reactions with air
Iodine is not reactive towards with oxygen
or nitrogen
. However, iodine does react with ozone, O3 to form the unstable yellow I4O9.
Reactions with halogens
Iodine reacts with fluorine at room temperature to form the iodine(V) pentafluoride. At 250oC the same reaction yields iodine(VII) heptafluoride. With careful control of the reaction conditions, (-45oC, suspension of CFCl3), it is posible to isolate the iodine(III) fluoride.
I2(s) + 5F2(g) --> 2IF5(l)
I2(s) + 7F2(g) --> 2IF7(g)
I2(s) + 3F2(g) --> 2IF3(s)
Iodine reacts with bromine to form the very unstable interhalogen species iodine(I) bromide.
I2(s) + Br2(l) --> 2IBr(s)
Iodine reacts with chlorine at -80oC with excess liquid chlorine to form iodine (III) chloride.
I2(s) + 3Cl2(l) --> I2Cl6(s)
Iodine reacts with chlorine in the presence of water to form iodic acid.
I2(s) + 6H2O(l) + 5Cl2(g) --> 2HIO3(s) + 10HCl(g)
 
Reactions with acids
Iodine reacts with hot concentrated nitric acid to form iodic acid. The iodic acid crystallizes out on cooling.
3I2(s) + 10HNO3(aq) --> 6HIO3(s) + 10NO(g) + 2H2O(l)
 
Reactions with bases
Iodine reacts with hot aqueous alkali to produce iodate, IO3-. Only one sixth of the total iodine is converted in this reaction.
3I2(g) + 6OH-(aq) --> IO3-(aq) + 5I-(aq) + 3H2O(l)

Occurrence of Iodine

Iodine naturally occurs in the environment chiefly as dissolved iodide in seawater, although it is also found in some minerals and soils. The element may be prepared in an ultrapure form through the reaction of potassium iodide with copper(II) sulfate. There are also a few other methods of isolating this element. Although the element is actually quite rare, kelp and certain other plants have the ability to concentrate iodine, which helps introduce the element into the food chain as well as keeping its cost down. 

Iodine is found in the mineral Caliche, found in Chile, between the Andes and the sea. It can also be found in some seaweeds as well as extracted from seawater, however extracting Iodine from the mineral is the only economical way to extract the substance. 

Extraction from seawater involves electrolysis, the brine is first purified and acidified using sulphuric acid and is then reacted with chlorine. An Iodine solution is produced but it is yet too dilute and has to be concentrated. To do this air is blown into the solution which causes the iodine to evaporate, then it is passed into an absorbing tower containing acid where sulfur dioxide (SO2) is added to reduce the iodine, the solution is then added to chlorine again to concentrate the solution more, the final solution is the Iodine at a level of about 99%. 

Another source is from kelp. This source was used in the 18th and 19th centuries but is no longer economically viable. 

In 2005, Chile was the top producer of iodine with almost two-thirds world share followed by Japan and the USA reports the British Geological Survey.

Isotopes of Iodine

There are 37 isotopes of iodine and only one, I-127, is stable.
Iodine-123 and iodine-125 are used in medicine as tracers for imaging and evaluating the function of the thyroid. 

127I [74 neutrons]
Abundance: 100%
Stable with 74 neutrons 

129I [76 neutrons]
Abundance: synthetic
Half life: 1.57 x 107 years [ beta- ]
Decay Energy: 0.194 MeV
Decays to 129Xe.
Excesses of stable 129Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas which created the dust and gas from which the solar system formed.
129I (half-llife 15.7 million years) is a product of cosmic ray spallation on various isotopes of xenon in the atmosphere, in cosmic ray muon interaction with tellurium-130, and also uranium
and plutonium fission, both in subsurface rocks and nuclear reactors. Nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests have now swamped the natural signal for this isotope. 129I was used in rainwater studies following the Chernobyl accident. It also has been used as a groundwater tracer and as an indicator of nuclear waste dispersion into the natural environment. 

131I (radioiodine) [78 neutrons]
Abundance: synthetic
Half life: 8.02070 days [ beta- ]
Decay Energy: 0.971 MeV
Decays to 131Xe.
Has been used in treating cancer and other pathologies of the thyroid gland.

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